I was playing Platinum, biking the route between Solaceon Town and HeartHome City. I had long ago beaten the Joggers there, and I've talked to them a million times, run into them a million times, etc.
But today(Saturday for me), I accidentally ran into a Jogger, and he challenged me to a Pokemon battle! The same happened on a different route.
Was it because it was the weekend? They've never done that before, and I've been going through the route very often for about a year(no joke).